1. Field of the Invention
The invention relates to a method for making a conductive film, more particularly to a method for making a conductive film of carbon nanotubes
2. Description of the Related Art
In view of the wide application of a liquid crystal displayer, the development of a transparent conductive material is always a popular research subject. The transparent conductive film which can be applied in the displayer and the touch panel should have the following characteristics: 1) Both of the light transmittance and the conductivity thereof in the visible light range should be high; 2) A film having a smooth surface can be made thereby and a plasma processing environment can be sustained; 3) It can be easily etched so as to form a predetermined pattern; 4) It can be homogenized in a large area; 5) the production cost thereof is low; and 6) it is nonpoisonous and can be recycled. Indium tin oxide has low film specific resistance and a visible light transmittance ranging from 80 to 90%, and thus is presently a main raw material for the transparent conductive film. However, since Indium contained in the Indium tin oxide material is a rare metal, the supply thereof is unstable and the cost thereof is increasing. Therefore, the development of an alternative material for replacement of Indium tin oxide become a main subject. Additionally, since the film made of indium tin oxide has a flexibility that is not sufficient for the application of the touch panel and the flexible panel, it has the disadvantages such as relative inferior durability and relative low liability.
In view of the aforesaid disadvantages of the indium tin oxide material, a carbon nanotube material has become a popular replacement for the indium tin oxide material because the carbon nanotube material has superior optical, electrical, magnetic, and mechanical properties. A conductive film made of single-walled carbon nanotubes (referred to as “SWNTs” hereinafter) has been developed in a commercial application.
The conductive film of the SWNTs is made primarily by a filtration method or a spraying method. The filtration method includes the steps of: synthesizing the SWNTs by a laser method, acid washing the SWNTs by a concentrated nitric acid solution, adding the SWNTs into a solvent containing a specific surfactant to form a dispersion of the SWNTs, filtering the dispersion of the SWNTs onto a filtration membrane to form a film of the SWNTs on the membrane, spreading the film of the SWNTs together with the filtration membrane onto a transparent substrate, and removing the filtration membrane using acetone so as to produce the film of the SWNTs (see, for example, “Transparent Conductive Carbon Nanotube Films” by Z. Wu etc., Science 2004, 305, 1273, and “Effect of SOC12 Treatment on Electrical and Mechanical Properties of Single-Wall Carbon Nanotube Networks” by U. Dettlaff-Weglikowska etc., J. Am. Chem. Soc., 2005, 127, 5125-5131).
The spraying method for making the conductive film of the SWNTs includes the steps of: dispersing a predetermined amount of the SWNTs into a solvent containing a specific surfactant to form a dispersion of the SWNTs, centrifuging the dispersion of the SWNTs, and spraying the supernatant solution of the SWNTs on a substrate of polyethylene terephthalate following by several rinsings in deionized water and drying so as to make the conductive film of the SWNTs (See, for example, “Effect of Acid Treatment on Carbon Nanotube-Based Flexible Transparent Conducting Films”, J. Am. Chem. Soc., 2007, 129, 7758-7759).
Although various methods for making the conductive film of the SWNTs have been researched and developed for replacement of the indium tin oxide film, there is still a need in the art to provide a method for making a conductive film of carbon nanotubes having good performance in transparence, conductivity, and flexibility in a simple and low-cost manner.